A liquid electrographic imaging system includes an imaging substrate onto which a developer liquid is delivered to develop a latent image. The imaging substrate may be a permanent image receptor or alternatively, may be a temporary image receptor. A liquid electrographic imaging system may be an electrostatic system having a dielectric material as the imaging substrate, or may take the form of an electrophotographic system having a photoreceptor as the imaging substrate. In an electrostatic system that makes use of a dielectric material, the latent image can be formed by selectively charging the dielectric substrate with an electrostatic stylus. In an electrophotographic system, the photoreceptor includes a photoconductive material that is uniformly charged, for example, with a corona charging device. A latent image can be formed on the photoreceptor by selectively discharging the photoreceptor with a pattern of radiation. A liquid electrophotographic imaging system with a photoreceptor will be discussed for purposes of example.
Several techniques have been developed over the years to adapt electrographic techniques to use multiple colors, such as disclosed in U.S. Pat. Nos. 3,832,170 (Nagamatsu et al.); 4,578,331 (Ikeda et al.); 4,728,983 (Zwadlo et al.); and 4,877,698 (Watson et al.). Color printers use three primary colors, typically cyan, magenta and yellow, and in addition, optionally, black. The toner may be a dry powder or a liquid.
The liquid ink (also referred to as a liquid toner or liquid developer) for an electrophotographic printing system normally includes solids or toner particles dispersed in a liquid carrier or solvent at a predetermined ratio. These two components deplete at different rates from the ink during printing. The relative component depletion rates are dependent on the percent coverage of the image produced by the imaging system and on other factors. For example, as the solids get transferred to the receptor material, the ratio of liquid carrier to solids increases, necessitating the addition of more solids to the ink. In images where the percent coverage is relatively low, liquid carrier is lost from the receptor material at a rate faster than the solids are depleted from the ink, requiring additional liquid carrier to be added to the ink to maintain the required ratio of solids to liquid carrier. Various systems have been made to deliver ink solids and liquid carrier to electrophotographic printers.
U.S. Pat. No. 5,208,637 (Landa) discloses a liquid toner replenishment system for an electrophotographic printer that does not have a separate liquid carrier replenishment system. When the density of the liquid toner in the mixing reservoir is below a first predetermined level, a motor is activated to add a measured amount of toner concentrate from a container to the respective toner reservoir, thereby increasing the toner particle concentration to the required level. Since the proportion of toner particles to liquid carrier in the container is less than the concentration carried out of the toner reservoir, an excess of liquid carrier is effectively added to the reservoir in excess of that removed by printing. This process causes the liquid level in the tank to rise slightly each time the toner concentrate is added. No mechanism is provided for adding liquid carrier to the reservoirs in the event the concentration of toner particles exceeds a predetermined level.
U.S. Pat. No. 3,973,699 (Cook) discloses a liquid dispensing apparatus utilizing a double-acting piston. When the level of liquid developer drops to a predetermined level, a drive unit is energized and the liquid developer is forced from the cartridge. The pressure drop caused by movement of the piston is employed to draw waste developer from a sump into a chamber within the cartridge. The liquid dispensing apparatus of Cook does not provide separate sources of ink concentrate or liquid carrier in order to maintain the required ratio of those components in the developer tray.
U.S. Pat. No. 3,876,282 (Schon et al.) discloses a container for supplying liquid developer in an electrophotographic copier. The container of Schon holds both developer concentrate and developer separated by a sliding separating wall. Developer concentrate is displaced into the correspondingly enlarged liquid developer chamber by movement of the sliding separating wall. Although the concentration of the liquid developer can be increased by the addition of developer concentrate, the container of Schon does not provide a source of liquid carrier to lower the concentration of the liquid developer when necessary.